1. Field of the Invention
The present invention relates to a battery charger suited for charging a battery, such as a nickel metal hydride battery, which emits high heat while being charged.
2. Description of the Related Art
Presently, a chargeable battery which can be repeatedly used for the power supply of, for example, a power tool is used. A nickel cadmium battery is popular as a battery for the power tool, and a battery charger for quickly charging a battery by applying high current to the battery is used. Specifically, the battery is quickly charged in about 20 minutes and a power tool can be continuously used by changing a battery to a battery which has been charged.
The inventor of the present invention studied improving the performance of a power tool by using a nickel metal hydride battery as a battery therefore. Although the nickel metal hydride battery can increase a capacity compared to a nickel cadmium battery, it generates high heat while being charged. If the temperature of the battery becomes high by the generated heat, the electrodes and separator of the cell within the battery deteriorate and battery life is shortened. Due to this, it is impossible to quickly charge the nickel metal hydride battery with high current as done for the nickel cadmium battery stated above.
Further, the nickel metal hydride battery is more sensitive to overcharge than the nickel cadmium battery and, if overcharged, the battery life is shortened. Due to this, it is necessary to avoid overcharging the battery. To avoid overcharge, in case of equipment which does not change one battery to another, 100% charge can be conducted by:integrating charging current and discharging current, and by charging the battery based on the integral value. In case of charging a plurality of batteries while changing one battery to another for use in a power tool as stated above, however, it is difficult to charge them up to 100% capacity without causing overcharge.
Moreover, according to the conventional technique the battery charger for use in a power tool has enough charging capacity to continuously charge about two batteries with a view to providing a power supply circuit at low cost. Due to this, if three or more batteries are to be continuously charged, charging current is considerably lowered by a protection device. As a result, if continuous charge is conducted, it takes a longer time to charge the third or the following batteries.
Further, the inventor of the present invention contrived a method for detecting the completion of 100% charge from a map in which the absolute temperatures of a battery and temperature rise values are mapped. With this method, however, it sometimes occurs that the completion of 100% charge is determined after 100% charge is over, depending on conditions.
In addition, it is required to more accurately detect that a battery is charged to a desired capacity level by the method using the map.
To accurately charge a plurality of types of batteries by the method using the map, maps for the plural types of batteries are required, respectively, with the result that a large storage memory is required for control purposes.
Even if a method for detecting full charge from the map is used, it is difficult to charge batteries in the same manner. This is because various battery packs are employed for various voltages. Namely, a high voltage battery pack contains a number of battery cells and heat tends to accumulate within the pack. Conversely, a low voltage battery pack contains less battery cells and heat inside tends to diverge.
Also, since the nickel metal hydride battery has different temperature rise characteristics according to the residual battery capacity, it is difficult to charge the nickel hydride battery of varied residual capacities using the above-stated map.
Furthermore, if the quantity of current is controlled by detecting battery temperature, it is difficult to charge the battery using the map since the battery temperature changes in a different manner according to environmental temperature.
Although some conventional battery chargers can switch long-time charge to short-time charge and vice versa, the life of a battery is considerably shortened during short-time charge. In addition, the battery charger according to the conventional technique cannot accurately charge a battery to desired capacity.
If a battery pack consisting of a plurality of battery cells is used a number of times by repeatedly charging and discharging them, the capacities of the respective battery cells vary and the performance of the battery pack deteriorates. To deal with deterioration in performance, auxiliary charge is often conducted after completion of ordinary charge, to fully charge the respective battery cells by carrying charging current of about 0.1 C for one to two hours.
Also, the capacity of a nickel metal hydride battery or nickel cadmium battery which has been 100% charged sharply decreases to about 90% by self discharge which takes place right after battery charge is stopped. Thereafter, the battery capacity gradually decreases by self discharge. To deal with a sharp decrease in capacity right after the completion of charge, trickle charge is sometimes conducted in which current of about 0.02 C is continuously carried after the completion of charge.
In the auxiliary charge and trickle charge stated above, average low current is applied to a battery by carrying pulse current. That is to say, the temperature of the battery rises when charge is completed and the capacity of the battery increases to a maximum. At this time, even if quite low current is continuously carried to the battery, the battery cannot be efficiently charged with the current. Considering this, pulse charge is adopted in which slightly high current is carried in a short time and then intermitted for a while.
In the battery charger according to the conventional technique, however, since pulse current is carried in specified cycles, decrease in battery temperature after the completion of charge is delayed. A state in which the battery temperature is high right after the completion of charge continues, thereby adversely influencing the battery life. Besides, while the battery is kept in a high temperature state, charging efficiency is low. To deal with this disadvantage, the temperature of the battery may be lowered by making the pulse current cycle longer. If the charging current cycle, i.e., pulse cycle is made longer to the extent that the battery can be quickly cooled down, self discharge takes place in the battery while the current is not applied to the battery. Due to this, a variation in battery capacity is large when a user takes out the battery from the battery charger. The purpose of auxiliary charge and trickle charge cannot, thereby, be attained.